Hydrogen is an important gas for the operation of a wide range of industrial applications. One particular field of application is oil refineries, where demand for hydrogen is constantly increasing as a direct result of the steady increasing demand on refinery capacity. Refining units used for operations such as hydro-denitrogenation, hydrocracking and dearomatisation require the addition of hydrogen. The hydrogen requirements in these refining units may for instance be met by hydrogen produced in a plant nearby specifically devoted to the refinery needs.
In particular, during hydro-denitrogenation (HDN) hydrogen is added to an oil feedstock stream and is passed over a hydrotreating catalyst in order to convert nitrogen- and sulfur-containing compounds to ammonia and hydrogen sulfide. The effluent from the hydrotreating unit may be passed to a separating vessel, where a gaseous phase containing ammonia and hydrogen sulphide is separated from a liquid phase. The nitrogen- and sulfur-free liquid phase is then be passed over a hydrocracking unit, whereby the risk of poisoning the nitrogen-sensitive hydrocracking catalyst is eliminated. By hydrocracking the liquid phase is converted into lighter products which are of higher value for instance gasoline.
Normally, as generally described in EP-A-0354 623, the ammonia containing stream separated during the HDN stage is simply discharged and subjected to purification and sulfur recovery. In other instances, such as in our WO-A-0238704, ammonia may be added to the liquid phases of the one or more catalyst beds in a hydrocracking unit in order to improve product selectivity and reduce hydrogen consumption.
More generally ammonia is also known as a hydrogen carrier and accordingly it has been suggested to decompose or crack ammonia in order to produce nitrogen and hydrogen. U.S. Pat. No. 5,976,723 describes a method and materials for the cracking of ammonia to produce hydrogen, which is suitable for use in internal combustion engines. WO-A-01/87770 mentions the possibility of autothermally decomposing ammonia to produce hydrogen, where the hydrogen can be used in refinery processes such as hydrocracking, hydrotreating and hydroisomerization.
The utilization of ammonia in connection with reforming and subsequent downstream processes for production of hydrogen from synthesis gas, particularly in larger plants capable of producing up to 120 000 Nm3/h hydrogen or even more has been precluded due to the often inevitable presence of sulfur in the ammonia stream. Sulfur acts as a poison for downstream catalysts such as nickel-based reforming catalyst as well as water-gas shift catalysts used for the enrichment of the synthesis gas in hydrogen.
It would be desirable to be able to utilize ammonia containing sulfur and which is emitted from chemical processes such as hydro-denitrogenation in oil refineries, as a valuable hydrogen source instead of simply disposing it, as disposal of ammonia requires transportation in specially designed containers and thus it represents a complicated and highly expensive process.